This invention relates to an electro-optical light modulator suitable for use in light communication systems, light information processing systems, light wavelength multiplex signal transmitting apparatus, light wavelength separating switches and the like.
In light communication systems and information processing systems utilizing light for the purpose of increasing transmission capacity, methods have been tried to increase the speed of modulation of the light and to use light fibers operating in a broader bandwidth for increased signal speed. However, there are practical limits to these methods which make it difficult to obtain a sufficiently large transmission capacity.
As a method of further increasing the transmission capacity, light wavelength multiplex transmission systems have been developed. In the transmitting apparatus of these prior systems, light waves having different wavelengths are independently modulated and then synthesized. To effect such modulation, light waves emitted by a plurality of semiconductor lasers, oscillating at different wavelengths, are independently modulated and then synthesized; or a plurality of laser light waves having different wavelengths are modulated with different light modulators and then synthesized.
According to the first of these two prior methods, since light sources of a number equal to the number of wavelengths to be synthesized are necessary, the transmitting apparatus of the light wavelength multiplex signal is extremely expensive. According to the latter method, it is necessary to use light modulators of a number equal to the number of wavelengths to be multiplexed. In order to synthesize modulated light waves having different wavelengths, it is necessary to use an optical system for this purpose which results in increasing the size of the apparatus.
It is also known to use photo-lithographic techniques to form a plurality of light sources and light guides for modulating and synthesizing a plurality of light waves having different wavelengths. However, in this method, application and emission of light are difficult and transmission loss is so large that the method is not practical.
Typically, a light wavelength multiplex transmission system requires a terminal device which independently modulates a plurality of light waves having different wavelengths and simultaneously transmits the modulated light waves; and, a receiving apparatus that includes a means for separating received light waves into different wavelength components and for coupling these components into different light paths. It is also necessary to provide a switch which electrically effects switching between different light paths, so as to couple, at a high speed, the light paths to a plurality of terminal devices.
Among known methods for performing wavelength separating functions are an interference filter comprising a plurality of superimposed dielectric films, a prism-shaped light analyzer utilizing the property of variation of refractive index with wavelength, and a light analyzer utilizing a diffraction grid. In all of these devices, however, the light paths are fixed for all wavelengths so that it is necessary to use another device for effecting switching between light paths. In addition, it is also necessary to provide a device for switching light paths of different wavelengths. Increases in the number of such devices not only complicate the apparatus but also decrease the reliability thereof. In order to miniaturize and decrease the cost of the apparatus, it is advantageous to decrease the number of the devices to a minimum.